Add issue forms and clarify source build requirements

[kwisatzh]

Summary

• Adapt the collector issue forms for Fleetdiff bug reports, feature requests, and questions.
• Ask for synthetic reproductions and direct security reports to private reporting.
• Update current-source instructions for the Go 1.26 minimum introduced by the OTLP dependency update. Published v0.5.0 binaries are unchanged.

Validation

• Parsed all three YAML forms and checked unique field identifiers.
• Checked demo shell syntax and help output.
• git diff --check.